1. Field of the Invention
This invention relates generally to a power supply circuit for discharge machining, and more particularly to a power supply circuit constructed so as to apply a d-c power voltage to a discharge gap between a discharge machining electrode and a workpiece via a switching means, in which discharge machining speed is improved by reducing the off time of the switching means by quickly discharging the energy stored in the lead wires after the switching mans is turned off.
2. Description of the Prior Art
FIG. 4 shows the construction of a power supply circuit for discharge machining of a conventional type. FIG. 5 is a diagram of assistance in explaining the waveforms generated in the circuit shown in FIG. 4.
In FIG. 4, numeral 1 refers to a workpiece; 2 to a discharge electrode; 3 to a d-c power supply from which a d-c voltage is impressed across the workpiece 1 and the discharge electrode 2. Numeral 4 refers to a transistor constituting a switching means for applying a voltage across the workpiece 1 and the discharge electrode 2. Numeral 41 refers to a resistor; 42 to a diode; symbol L.sub.o to the inductance of lead wires; G1 to a gate voltage of the transistor 4; I to a current; E to a voltage; R.sub.g to a gap resistance, respectively.
First, a voltage such as G1 shown in FIG. 5, is applied to the gate of the transistor 4. As the gate voltage G1 is applied, the transistor 4 is turned on, causing a d-c voltage E3 to be applied across the workpiece 1 and the discharge electrode 2 from the d-c power supply 3. At this time, control is effected so that the discharge electrode 2 gradually comes near the workpiece 1, and that the voltage E3 across the workpiece 1 and the discharge electrode 2 is kept high until discharge begins, and decreased to a lower level after discharge begins.
When the transistor 4 is turned off, the energy stored in the inductance L.sub.o of the lead wires by a current I1 flowing in the discharge gap between the workpiece 1 and the discharge electrode 2 turns into a current I2, which in turn flows through a gap resistance R.sub.g in the discharge gap via the diode 42. The gap resistance R.sub.g is determined by a working liquid, machining chips a stray capacity, etc. Although the energy stored in the inductance L.sub.o of the lead wires is, as a rule, reduced exponentially through the gap resistance R.sub.g because of the existence of the diode 42, the OFF period during which the transistor 4 is kept in the off state has to be made longer because waiting time is needed until the current I2 becomes zero. That is, if the voltage E3 exists in the discharge gap due to the energy stored in the inductance L.sub.o or the energy stored in the stray capacities C.sub.o and C.sub.G, there arises a problem of the difficulty in removing machining chips, free carbon and other floating substances out of the discharge gap. If the d-c voltage E3 is applied across the workpiece 1 and the discharge electrode 2 in the presence of these floating substances in the discharge gap, discharge is unwantedly concentrated, leading to arc discharge. Generation of arc discharge causes the working surface to be roughened, accelerating the consumption of the electrode, or causing discharge machining to be interrupted. To cope with this, the OFF period during which the transistor 4 is kept in the off state has to be made longer, as noted earlier. The longer OFF period, however, leads to an unwanted problem of lowered machining speed.